Batch sorption of sulfamethazine was conducted using five soils with organic carbon (OC) contents ranging from 0.1% to 3.8% and solution pHs ranging from 5.5 to 9. Sorption of sulfamethazine was found to be impacted by OC, soil surface area and soil solution pH, with higher K(d) values for soils with higher OC and lower K(d) values as the pH increased. However, OC was found to be the more dominant parameter. Linear partition coefficients at pH 5.5 were found to be 0.58+/-0.17 Lkg(-1) for soil with 0.1% OC and 3.91+/-0.36 Lkg(-1) for soil with 3.8% OC. At pH 9, the K(d) values were found to decrease by more than 50% to 0.23+/-0.06 Lkg(-1) (soil with 0.1% OC) and 1.16+/-0.05 Lkg(-1) (soil with 3.8% OC). Hydrophobic sorption was probably involved for pH<7.4 (pK(a,2)=7.4 for sulfamethazine) due to the non-ionized form of sulfamethazine while surface sorption was probably involved for pH>7.4 due to the ionized form of sulfamethazine. This was confirmed by regressing the estimated sorption coefficients of cationic, uncharged, and anionic species against the soil properties. A stepwise linear regression model incorporating the anionic fraction of sulfamethazine ionization and soil properties were developed and were found to estimate the K(d) values of other studies using soils of different pH and soil properties.